THE CIRCULATION OF THE BLOOD. 



271 



return of the blood into the ventricle (Fig. 116); during the con- 

 traction they are directed into the artery (Fig. 117). In the former 

 position they are shut ; in the latter, they are open. 



The left auriculo- ventricular opening is provided with a similar 

 though better developed fi- 

 brous ring and membran- 

 ous valve. It is, however, 

 subdivided into but two 

 portions or cusps, and is 

 therefore termed the bi- 

 cuspid valve, or, from its 

 fancied resemblance to a 

 bishop's mitre, the mitral 

 valve. The general ar- 

 rangement, connections, 

 and mode of action of this 

 valve are similar in all re- 

 spects to those of the tricus- F IG. n 8. VALVES or THE HEART, i. Right 

 j i rpn /- r auriculo-ventricular orifice, closed by the 



pid valve. The orifice of tricuspid valve 2 Fibrinous ring . 3 y Le f t 



the aorta is also Surrounded auriculo-ventricular orifice, closed by the 



by a ring of fibrous tissue mi ' ral vah ?- 4- Fibrinous ring. 5. Aortic 



, , . , T , , T orifice and valves. 6. Pulmonic orifice 



to which are attached three and valves . 7 , 8 , 9 . Muscular fibers.- 



Semilunar Or pocket- shaped (Bonamy and Beau.) 



valves (Fig. 113), which in 



their arrangement, connections, and mode of action are similar in all 

 respects to those at the orifice of the pulmonary artery. The anatomic 

 relations of the cardiac orifices one to the other and the appearance 

 presented by the valves when closed are repre- 

 sented in Fig. 119. 



The Heart Muscle-fibers and Their Ar- 

 rangement. The muscle-fibers of the heart, 

 though transversely striated and nucleated, 

 differ in shape and arrangement from those 

 found in any other situation. The individual 

 fiber is short and broad and usually divided at 

 one or both ends| By this means the fibers are 

 united not onlv longitudinally, but laterally. 

 (See Fig. ii9-j\ The fibers are devoid of a 

 sarcolemma and united one to the other by a 

 cement material.^ The entire musculature is 

 permeated and supported by connective tissue 

 which is so arranged as to group the fibers in 

 bundles or fasciculi of varying size. 



The arrangement of the muscle bundles is quite complicated and 

 in accordance with the functions of the individual portions of the 



FIG. 119. MUSCLE- 

 FIBERS FROM THE 

 HEART OF A MAM- 

 MAL. (Landois 

 and Stirling.) 



